Spark plug

ABSTRACT

In a spark plug  100,  the outer diameter of a tool engaging portion 1 e  formed on a main metal member  1  is not larger than 14 mm. An insulator  2  is provided with a expanded diameter portion ( 2   e ) positioned within the main metal member  1  and projecting radially outward from an outer circumferential surface of the insulator, and an intermediate trunk portion ( 2   g ) adjacent to an axial front side of the expanded diameter portion ( 2   e ) and having a cylindrical outer circumferential surface. The inclined portion ( 2   j ) has a linearly inclined outer circumferential surface formed in a position so as to connect the expanded diameter portion ( 2   e ) and intermediate trunk portion ( 2   g ), and formed so that in orthogonal projections with respect to a plane of projection parallel to an axis O, the diameters become smaller from the side of the expanded diameter portion ( 2   e ) toward that of the intermediate trunk portion ( 2   g ). In the position in which an extension of the outer circumferential surface of the inclined portion ( 2   j ) and that of the outer circumferential surface of the intermediate trunk portion ( 2   g ) cross each other, a built-up portion ( 2   k ) is formed so that a valley-like space defined by these two extensions is filled.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to a spark plug for internal combustionengines, and a method of manufacturing the same.

[0003] 2. Description of the Related Art

[0004] A spark plug used for the ignition of an internal combustionengine, for example, a gasoline engine for automobiles, etc., has astructure including an insulator on an outer side of a centralelectrode, a main metal member on an outer side of the insulator, and anearth electrode forming a spark discharge gap between the earthelectrode and central electrode and fixed to the main metal member. Sucha spark plug is fixed to a cylinder head of an engine via an attachingscrew portion formed on an outer circumferential surface of the mainmetal member, and then put to use. Since the portion of the electrodewhich forms a spark discharge gap is exposed to a combustion gaseousmixture during an operation of the engine, the temperature of thisportion becomes extremely high.

[0005] With an increase in recent years in output from an internalcombustion engine used for automobiles, etc., the areas occupied bysuction and exhaust valves in a combustion chamber have also increased.This makes it necessary to miniaturize the spark plug used to ignite agaseous mixture, and the temperature of the interior of a combustionchamber tends to become increasingly higher due to a supercharging unit,such as a turbocharger. In order to secure the lifetime of a spark plugsatisfactorily even in such a severe condition of use, it is necessaryto design the spark plug so as to provide good heat radiation (reductionof heat). The heat of the spark plug is discharged through variouspaths. Especially, a path extending from an insulator to a cylinder headvia an attaching screw portion of a main metal member has a high heatflow, and plays an important role in securing reliable heat radiation.Under these circumstances, an attempt to improve the heat radiationperformance of a spark has been made by further increasing the length(screw reach) of this attaching screw portion. When a screw reach isincreased, the length of the insulator provided within the main metalmember naturally also increases.

[0006] Concerning a main metal member of a spark plug, the demand hasincreased for the reduction of the sizes of portions other than anattaching screw portion, in particular reduction of the hexagonalportion (tool engaging portion) which projects outward from a positionhigher than that of the attaching screw portion and which is provided sothat a wrench can be engaged therewith. The situation causing thisdemand will be described below. The space above the cylinder head haslost superfluidity due to employing a direct ignition system in whicheach ignition coil is fixed directly to an upper portion of a sparkplug, and the diameter of the plug hole has decreased due to an increasein the areas occupied by the above-mentioned valves. As a result, thedistance between opposite sides of the hexagonal portion necessarilydecreases to 14 mm or smaller, and since only a hexagonal portion notsmaller than 16 mm could be secured in a related art spark plug, adecrease in the diameter of an insulator is further required.

[0007] 3. Problems to Be Solved by the Invention

[0008] When an impact is exerted on an insulator, in which the reductionof the diameter in the direction crossing the axis of a spark plug andthe enlargement in the axial direction of the length thereof wereeffected as mentioned above, problems such as bending and the like ofthe insulator become liable to occur. In a spark plug of a generalstructure, a flange type projecting portion called an expanded diameterportion is provided on an outer circumferential surface of theinsulator, and a main metal member is joined with the expanded diameterportion by clamping with the rear end portion of the main metal memberdirected toward this expanded diameter portion. A valley-like section isformed in the inner circumferential edge of the end surfaces of thisflange-like expanded diameter portion extending in the circumferentialdirection, and this valley-like section tends to receive stressconcentration in particular due to a notch effect. Since this expandeddiameter portion is formed in a position comparatively close to ahexagonal portion (tool engaging portion) of a main metal member, theexpanded diameter portion is liable in particular to be influenced bythe reduction in the diameter of the hexagonal portion. Therefore, whenexcessive torsion is exerted on an insulator via the main metal memberduring the driving of an attaching screw portion into an engine head,or, when an impact is imparted to a plug head portion during the fixingof a spark plug to the engine head, especially, problems, such asbending and the like of the insulator of a small diameter and a largelength become liable to occur due to both the increase in bending momentand a decrease in the cross-sectional area of the insulator.

SUMMARY OF THE INVENTION

[0009] Therefore, an object of the present invention is to provide aspark plug capable of effectively preventing the insulator from beingbent during a spark plug fixing operation or when an impact, etc., isimparted to the spark plug due to external causes other than the sparkplug fixing force despite the use of an insulator having a diameterwhich is reduced in accordance with a decrease in the dimensions of atool engaging portion thereof.

[0010] The above object of the present invention has been achieved byproviding a spark plug including a shaft type central electrode 3, ashaft type insulator 2 covering an outer side of the central electrode3, a main metal member 1 which is formed cylindrically so as to beopened at both ends thereof, and which is disposed on an outer side ofthe insulator 2, and an earth electrode 4 combined at one end thereofwith the main metal member 1 and opposed at the other end thereof to thecentral electrode 3 so as to form a spark discharge gap g, said sparkplug having a center axis O in the axial direction of the insulator 2,wherein:

[0011] the main metal member is provided on an outer circumferentialsurface thereof with an attaching screw portion 7 and a tool engagingportion le for turning the screw portion 7 into a threaded hole in aninternal combustion engine, the interval between opposite sides Σ; ofthe tool engaging portion 1 e being not greater than 14 mm,

[0012] the side in the axial direction of the insulator 2 on which thespark discharge gap g is formed is defined as the front side and theopposite side thereof is defined as the rear side, the insulator 2 beingprovided with a expanded diameter portion 2 e positioned in the mainmetal member 1 and projecting radially outward from the outercircumferential surface of the insulator 2, and an intermediate trunkportion 2 g having a cylindrical outer circumferential surface, formedadjacently in the axial direction to the front side of the expandeddiameter portion 2 e and engaged at its front end with a main metalmember-side engaging section 1 c formed on an inner circumferentialsurface of the main metal member 4,

[0013] an outer circumferential surface of an inclined portion 2 j formsa linear incline in a position connecting the expanded diameter portion2 e and intermediate trunk portion 2 g, such that the diameter of theinclined portion 2 j decreases from the side of the expanded diameterportion 2 e toward the intermediate trunk portion 2 g in orthogonalprojections parallel to said axis, a built-up portion 2 k being formedwhere the extension of the outer circumferential surface of the inclinedportion 2 j and that of the intermediate trunk portion 2 g cross eachother, such that a valley-like space defined by these extensions isfilled with the built-up portion 2 k.

[0014] Referring to the appended claims and the above description, thereference symbol following the name of each structural element is usedin reference to that added to a corresponding part illustrated in theattached drawings (FIG. 1, FIG. 2 and FIG. 3). These reference symbolsare added so as to only make the present invention easily understood,and do not limit at all the concept of each structural requirement forthe invention.

[0015] The present invention also relates to a spark plug in which thelength between opposite sides of the tool engaging portion le formed onthe main metal member 1 is not larger than 14 mm due to theabove-mentioned circumstances which cause the diameter of the insulator2 to be reduced. In the insulator 2 of this spark plug, a valley-likespace extending in the circumferential direction thereof is formedbetween the outer circumferential surface of the inclined portion 2 j,adjoining one edge of the expanded diameter portion 2 e, and theintermediate trunk portion 2 g. When torsion of a wrench occurringduring a spark plug installation operation and other impact forceoccurring during the same operation are imparted to the valley-likeportion to cause bending moment to work on the same, stressconcentration is liable to occur thereon. However, according to thepresent invention, the built-up portion 2 k is provided where theextension of the outer circumferential surface of the inclined portion 2j and that of the outer circumferential surface of the intermediatetrunk portion 2 g cross each other, so as to fill the valley-like spacetherewith. This can prevent the stress concentration from occurring, andenables the bending resistance etc. of the insulator to be improved by alarge margin.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1(a) is a general longitudinal sectional view showing a sparkplug constituting an embodiment of the present invention, and FIG. 1(b)is an enlarged sectional view of a principal portion thereof.

[0017]FIG. 2 is a drawing showing a definition of the length betweenopposite sides of a tool fixing portion.

[0018]FIG. 3 is a drawing describing size symbols for each part of aprincipal portion of the spark plug of FIGS. 1(a) and (b).

[0019]FIG. 4 is a drawing describing size symbols for each part shown inthe general drawing of the spark plug of FIG. 1.

[0020] FIGS. 5(a) to 5(d) are sectional view showing various examples ofthe shape of a leveled-up portion formed on an insulator in the sparkplug according to the present invention.

[0021] FIGS. 6(a) and 6(b) are drawings describing a glass sealingprocess.

[0022]FIG. 7 is a drawing describing an impact resistance test.

[0023]FIG. 8 is a graph showing the relationship between P/C, the valueof impact resisting angle and screw neck strength.

[0024]FIG. 9 is a graph showing the relationship between (S1/E)/(S2/D)and the value of impact resisting angle.

DESCRIPTION OF THE REFERENCE SYMBOLS

[0025]100 Spark Plug

[0026]1 Main Metal Member

[0027]1 e Tool Engaging Portion

[0028]1 g Fixing Seat Portion

[0029]2 Insulator

[0030]2 g Intermediate Trunk Portion

[0031]2 k Leveled-Up Built-Up Portion

[0032]2 j Inclined Portion

[0033]4 Earth Electrode

[0034] g Spark Discharge Gap

[0035]2 b Rear Side Section Of Main Body Portion

[0036] R1 Rounded Portion

[0037] R2 Rounded Portion

DETAILED DESCRIPTION OF THE INVENTION

[0038] An embodiment of the present invention will now be described withreference to an example shown in the drawings. However, the presentinvention shall not be construed as being limited thereto.

[0039]FIG. 1 is a longitudinal sectional view A of a spark plug 100 in amode of embodiment of the present invention, and an enlarged view B of aprincipal portion of the example. The spark plug 100 is provided with acylindrical main metal member 1, an insulator 2 fitted in an inner sideof the main metal member 1 so that a front end portion 2 i projectsoutward, a central electrode 3 provided in an inner side of theinsulator 2, and an earth electrode 4 joined at one end with the mainmetal member 1 by welding, etc. A spark discharge gap g is formedbetween the earth electrode 4 and central electrode 3. In the directionof an axis O of the insulator 2, a side on which the spark discharge gapg is formed will hereinafter be called the front side, and a sideopposite to this side the rear side.

[0040] The insulator 2 is provided in a central position on its axialcross section with a through hole 6 extending in the axial directionthereof, and a terminal metal member 13 is fixed in the rear end portionof the insulator 2 with the central electrode 3 also fixed in a frontend portion thereof. In the portion of the interior of the through hole6 between the terminal metal member 13 and central electrode 3, aresistor 15 is provided. Both end portions of this resistor 15 areelectrically connected to the central electrode 3 and terminal metalmember 13 respectively via conductive glass seal layers 16, 17. Theterminal metal member 13 is provided with a male screw engaging portion13 a on an outer circumferential surface of a front end portion thereof,and this engaging portion 13 a is fitted at a front end section thereofin the conductive glass seal layer 17 so as to thereby increase thebonding strength thereof.

[0041] The insulator as a whole is formed of an insulating material,such as alumina. An outwardly projecting cylindrical portion withexpanded diameter 2 e is formed in the shape of a flange on anintermediate section of the insulator 2 in the axial direction. Theinsulator 2 is formed at its section to the rear of the expandeddiameter portion to provide a rear side main body portion 2 b having adiameter smaller than that of the expanded diameter section. Acorrugation 2 c is provided on the outer circumferential surface of thisrear side main body portion 2 b. An intermediate trunk portion 2 g of adiameter smaller than that of the expanded diameter portion 2 e, and afront end portion 2 i of a diameter still smaller than that of theintermediate trunk portion 2 g are formed in the mentioned order infront of the expanded diameter portion 2 e.

[0042] On the other hand, the diameter in an axial cross section of thecentral electrode 3 is set smaller than that in an axial cross sectionof the resistor 15. The through hole 6 of the insulator 2 has a firstsubstantially cylindrical portion 6 a through which the centralelectrode 3 is inserted, and a second substantially cylindrical portion6 b formed to the rear (upper side in the drawing) of the first portion6 a with a diameter greater than that of the first portion 6 a. Theterminal metal member 13 and resistor 15 are held in the second portion6 b, and the central electrode 3 is inserted through the interior of thefirst portion 6 a. The central electrode 3 is provided on a rear endportion thereof with an electrode fixing projecting section 3 cprojecting outward from the outer circumferential surface thereof. Thefirst portion 6 a and second portion 6 b of the through hole 6 areconnect at the intermediate trunk portion 2 g, and a surface 6 c wherethe first and second portions 6 a, 6 b are connected and where theelectrode fixing projection 3 c of the central electrode 3 is receivedis formed to be a tapering surface or a rounded surface.

[0043] An outer circumferential surface (i.e., the front end section ofthe intermediate trunk portion 2 g) of a joint portion 2 h between theintermediate trunk portion 2 g and front end portion 2 i is formed as astepped surface, which is engaged with the projecting portion 1 c as amain body metal member-side abutting portion, which is formed on aninner surface of the main body metal member 1, via a ring shaped sheetpacking portion (not shown) to prevent the insulator from coming off inthe axial direction. On the other hand, a ring shaped wire packing 62engaged with the rear circumferential edge of the flange type expandeddiameter portion 2 e is provided between this and the inner surface of arear side opened portion of the main metal member 1. On the rear of thewire packing 62, another ring shaped wire packing 60 is provided via apacked bed 61 of talc or the like. The insulator 2 is pushed forwardinto the main metal member 1, and the open edge of the main metal member1 in this condition clamps inward toward the packing 60. As a result, aclamped portion Id is formed, and the main metal member 1 is fixed tothe insulator 2.

[0044] The main metal member 1 is formed cylindrically by using as a rawmaterial an iron material suitable for cold working, for example, lowcarbon steel, and a carbon steel wire and the like for cold forging asdefined in JISG3539 (1991), and constitutes a housing for the spark plug100. The housing 100 is provided on an outer circumferential surface ofthe front end portion of the spark plug with an attaching screw portion7 for fixing the spark plug 100 to an engine block (not shown). On thesection of the outer circumferential surface of the main metal member 1which is on the rear side of the attaching screw portion 7, acircumferentially extending flange type fixing seat portion 1 g isformed so as to project outward. To the rear side of the fixing seatportion 1 g, a tool engaging portion 1 e, at which a tool, such as aspanner or a wrench and the like used to turn the attaching screwportion 7 of the spark plug 100 into a threaded hole in a cylinder headis engaged, is formed via a thin-walled joint portion 1 h so that thetool engaging portion 1 e projects outward along the circumference ofthe spark plug 100.

[0045] As shown in FIG. 2, the tool engaging portion le includes aplurality of pairs of tool engaging surfaces 1 p parallel to the axis Oand to each other and formed so as to extend in the circumferentialdirection. An example of the tool engaging portion le shown in FIG. 2(a)has three pairs of such tool engaging surfaces 1 p, and is formed into aregular hexagonal cross sectional shape. An example shown in FIG. 2(b)is provided with twelve pairs of parallel tool engaging surfaces 1 p(which is also called a BIHEX shape) formed by superposing two righthexagonal shapes on each other (which is also called a HEX shape) bystaggering these two shapes from each other by 30° around the axis O. Inall of these examples, the length between opposite sides Σ of the toolengaging portion 1 e is expressed by the distance between opposite sidesof the contour of a right regular hexagonal cross section. In theabove-mentioned spark plug 100, the length between opposite sides Σ ofthe tool engaging portion 1 e is not larger than 14 mm.

[0046] The inclined portion 2 j has an outer circumferential surfacewhich is inclined linearly so as to extend from a side of the expandeddiameter portion 2 e toward that of the intermediate trunk portion 2 gas shown in FIG. 1(b) decreasing in diameter in orthogonal projectionswith respect to a plane of projection parallel to the axis O. In theposition in which the inclined portion 2 j and intermediate trunkportion 2 g are connected together, i.e., the inner circumferential edgeof one end surface of the expanded diameter portion, a valley-likeportion is formed extending in the circumferential direction. Thisvalley-like portion tends to receive, especially, stress concentrationdue to a notch effect. Especially, when the length between oppositesides Σ (FIG. 2) of the tool engaging portion le is not larger than 14mm as mentioned above, the axial cross-sectional area of the insulator 2necessarily decreases, so that the insulator 2 as a whole is shaped inan elongated manner and has a large total length with respect to theaxial cross-sectional area thereof. Therefore, when a wrench is engagedwith the tool engaging portion le with a large torsion for tighteningthe wrench applied thereto, or when a large impact force is exerted onthe rear side main body portion 2 b, etc., the bending moment working onthe insulator becomes liable to increase proportionally to the largelength thereof, and stress concentration on the valley type portionreadily occurs.

[0047] Therefore, in this spark plug 100, a built-up portion 2 k isformed in a position (valley type portion) of an intersection ofextensions 2 j′, 2 g′ of the outer circumferential surface of theinclined portion 2 j and that of the intermediate trunk portion 2 g sothat a valley-like space defined by the two extensions 2 j′, 2 g′ isfilled. Owing to this arrangement, the occurrence of excessive stressconcentration in the position in which the inclined portion 2 j andintermediate trunk portion 2 g are connected together can be avoided,and the bending resistance of the insulator 2 can be improved by a largemargin even when bending moment is exerted greatly thereon.

[0048]FIG. 5(a) shows the built-up portion 2 k on a further enlargedscale. Angles β1, β2 at which a contour line of the built-up portion 2 kand those of the inclined portion 2 j and intermediate trunk portion 2 gcross each other are evidently larger than an angle α (corresponding toa notch angle in a case where the built-up portion 2 k is not formed) atwhich the extensions 2 j′, 2 g′ of the contour lines of the inclinedportion 2 j and intermediate portion 2 g cross each other. As a result,stress on one recess of a smaller angle is scattered to two recesses oflarger angles, so that the bending resistance of the insulator isimproved. When in this embodiment recessed rounded portions R1, R2 areformed as shown in FIG. 5(b) in two points (first and second connectingpoints) A, c in which the outer circumferential surfaces of the built-upportion 2 k and the inclined portion 2 j and intermediate trunk portion2 g are connected to each other on the built-up portion 2 k, stressconcentration, which occurs in the first and second connecting points A,c, on the surfaces of the recesses is alleviated owing to provision ofthe rounded portions. This enables the bending resistance of theinsulator to be further improved.

[0049] In order to further improve the bending strength of the insulatorby forming the built-up portion 2 k, it is desirable that the shape ofthe built-up portion 2 k be set as follows. Namely, in orthogonalprojections with respect to a plane of projection parallel to the axis Oof the insulator, the outer circumference of the built-up portion 2 k isthe same as that of the surface defined by a reference line SL, i.e. astraight line which connects together the point A at which the outercircumferential surface of the built-up portion 2 k and that of theinclined portion 2 j are joined together and the point c at which theouter circumferential surface of the built-up portion 2 k and that ofthe intermediate trunk portion 2 g are joined together as shown in FIG.5, or bulged radially outward from the circumferences of reference lineSL. According to the present invention, the length between oppositesides Σ is not larger than 14 mm, so that the projection of the expandeddiameter portion in the radial direction thereof is limited. However,when the built-up portion 2 k is formed as mentioned above, the bendingresisting strength improving effect can be secured even though theradius of the leveled-up portion is reduced to a certain extent.Therefore, these techniques can be applied satisfactorily to an expandeddiameter portion 2 e which has a small radius.

[0050] More particularly, the above-mentioned mode of the built-upportion 2 k is specially effective in a case where the combining of thecentral electrode 3 and terminal metal member 13 with each other and theforming of the resistor 15 and conductive glass seal layers 16, 17 aredone by such a glass sealing process as will be described below. First,the central electrode 3 is inserted into the first portion 6 a of thethrough hole 6 of the insulator 2, and conductive glass powder and rawpowder of a resistor composition are then packed in order therein, theresultant materials being then subjected to preparatory compression toform a product as shown in FIG. 6(a) in which a first conductive glasspowder layer 26, a resistor composition powder layer 25 and a secondconductive glass powder layer 27 are laminated from the side of thecentral electrode 3 (front side). An assembly PA having a terminal metalmember 13 inserted from the above into the through hole 6 is thenformed. The assembly in this condition is inserted in a heating furnaceand heated to a predetermined temperature of 800 to 950° C. which is notlower than a softening point of glass. The terminal metal member 13 isthereafter press fitted axially into the interior of the through hole 6from the side opposite that of the central electrode 3, and each of thelayers 25 to 27 in a laminated state are pressed in the axial directionthereof. As a result, each layer is compress-sintered to be turned intothe conductive glass seal layer 16, resistor 15 and conductive glassseal layer 17 as shown in FIG. 6(b).

[0051] In this glass sealing process, a considerable axial pressure isapplied to the parts when each of the layers 25 to 27 in a laminatedstate is pressed axially with the terminal metal member 13 press fittedinto the through hole 6. When the pressing operation is carried out,this pressure is received by inserting the insulator 2 into an inserthole SH of a support base SB from a front side thereof, and supportingthe front side end surface of the expanded diameter portion 2 e on theopen outer circumferential edge of the insert hole SH. When during thistime the built-up portion 2 k is superposed on the circumferential edgeof the insert hole SH, the built-up portion 2 k is nibbled to cause thebuilt-up portion 2 k to be broken. Therefore, it is necessary to disposethe built-up portion 2 k so that the built-up portion 2 k as a whole ispositioned in the insert hole SH.

[0052] In this case, the pressing force is wholly received by theinclined portion 2 j positioned outside the built-up portion 2 k, sothat it is necessary to secure the width of the inclined portion 2 j ofnot lower than a predetermined level irrespective of the radius of theprojecting section of the expanded diameter portion 2 e. Therefore, whenthe radius of the projecting section of the expanded diameter portion 2e decreases, the width of the built-up portion is necessarily reduced.However, since the built-up portion 2 k of the above-mentioned shape cansatisfactorily secure its resistance to bending even when the width ofthe leveled-up portion is small, the present invention can also beapplied flexibly to a spark plug having a tool engaging portion le ofsmall length between opposite sides

[0053] As shown in FIG. 3, the width M of the outer circumferentialsurface of the inclined portion 2 j is desirably set not smaller than0.3 mm and not larger than 3 mm. When M is smaller than 0.3 mm, thesealing pressure cannot be stopped during the execution of a glasssealing step. When M exceeds 3 mm, the width of the built-up portion 2 kbecomes short, and the bending resisting strength improving effectbecomes insufficient. It is desirable that an angle Q between a plane APcrossing the axis O at right angles thereto and the outercircumferential surface of the inclined portion 2 j be not larger than60°. When Q exceeds 60°, a seal-pressing force cannot be receivedsufficiently during the execution of the glass sealing step.

[0054] In the above-mentioned orthogonal projection, J represents afirst intersection at which an extension of the cylindrical outercircumferential surface of the expanded diameter portion 2 e and that ofthe outer circumferential surface of the inclined portion 2 j cross eachother; n represents a second intersection at which an extension of theouter circumferential surface of the built-up portion 2 k and that ofthe outer circumferential surface of the intermediate trunk portion 2 gcross each other; and c represents a second connecting point at whichthe outer circumferential surface of the built-up portion and that ofthe intermediate trunk portion 2 g are connected together. In thedirection of the axis O, W represents a distance measured from a frontend surface 1 i of the fixing seat portion 1 g to the intersection n; Prepresents a distance measured from the second connecting point c to thesecond intersection n; and C represents a distance measured from thesecond intersection n to the first intersection J. It is desirable inthis condition that dimensional conditions of:

W>P≧0.5C  (1)

[0055] be satisfied.

[0056] The reference letter P represents the length of the overlap ofthe built-up portion 2 k over the intermediate trunk portion 2 g in thedirection of the axis O. The reference letter C corresponds to thelength in the direction of the axis O of a front end surface of theexpanded diameter portion 2 e including the inclined portion 2 j. When Pbecomes lower than 0.5C, the bending resistance improving effect becomesinconspicuous in some cases. On the other hand, P larger than W meansthat the built-up portion 2 k exceeds the front end surface 1 i of thefixing seat portion 1 g and extends forward in the direction of the axisO. As a result, the thickness in the radial direction of the main metalmember 1 lessens at a rear edge (so-called screw neck section) 7 f ofthe attaching screw portion 7 thereof, and torsional rupture strength(which will hereinafter be referred to as screw neck strength) of themain metal member 1 cannot be secured sufficiently in some cases. Forexample, when excessive tightening torque is exerted on the main metalmember during the fixing of the spark plug 100 to an engine head,inconveniences such as the breakage of the screw neck section 7 f occurin some cases.

[0057] It is desirable that the intermediate trunk portion 2 g of theinsulator 2 has an outer diameter F of not smaller than 5 mm and notlarger than 8 mm. When F is smaller than 5 mm, the bending resistingstrength of the insulator 2 cannot be secured sufficiently due to thesmall thickness. When F exceeds 8 mm, the thickness of the attachingscrew portion 7 becomes short, so that the screw neck strength decreasesin some cases. When an attaching screw portion 7 having a nominal sizeof M10 or M12 is used, this tendency is shown markedly.

[0058] The insulator 2 is provided as mentioned above with a main bodyportion 2 b having a diameter smaller than that of the expanded diameterportion 1 e, and a cylindrical outer circumferential surface, and formedadjacently to the rear of the expanded diameter portion 2 e with respectto the axis O. In the above-mentioned orthogonal projection shown inFIG. 4, K shall represent a position of a middle point of a sectorconnecting both ends of the cylindrical outer circumferential surface ofthe expanded diameter portion 2 e; T a position of the rear end in theaxial direction of the outer circumferential surface of the insulator 2;S the position of the front end in the direction of the axis O of thecylindrical outer circumferential surface of the intermediate trunkportion 2 g; E a distance measured in the direction of the axis O fromthe position T of the rear end to the position K of the middle point; Da distance measured from the position S of the front end to the positionK of the middle point; S1 an axial cross-sectional area (cylindricalouter circumferential surface portion, for example, position Y in thedrawing) of a rear side section of the main body portion 2 b; and S2(cylindrical outer circumferential surface portion, for example,position X in the drawing) an axial cross-sectional area of theintermediate trunk portion 2 g of the insulator. It is desirable that:

0.5≦(S1/E)/(S2/D)≦2  (2)

[0059] be satisfied.

[0060] S1/E represents in terms of a ratio with respect to the axialcross-sectional area S1 the length of the section (which willhereinafter be referred to as a rear projecting section) of the expandeddiameter portion le which exists on the rear side of the middle pointthereof, and a smaller value means that this section projects in a moreelongated manner). On the other hand, S2/D represents in terms of aratio with respect to the axial cross-sectional area S2 the length ofthe section of the expanded diameter portion le which exists on thefront side of the middle point thereof. When the ratio ((S1/E)/(S2/D))of these two values becomes excessively large to exceed two, the axialcross-sectional area S2 of the intermediate trunk portion 2 g becomesrelatively short, and the rear projecting section becomes extremelylong. As a result, when an impact, etc. is exerted on this section, alarge bending moment becomes liable to be imparted to a boundaryposition between the intermediate trunk portion 2 g and expandeddiameter portion 2E. Consequently, it becomes difficult to sufficientlysecure the bending resistance of the insulator 2. In the meantime, whenthe ratio becomes smaller than 0.5, the rear projecting section becomestoo short conversely, so that inconveniences, such as flushover becomesliable to occur.

[0061] In the examples of FIGS. 5(a) and 5(b), the built-up portion 2 kis formed in the orthogonal projection such that the outercircumferential surface thereof is substantially aligned with thecircumferences defined by the reference line SL. The built-up portion 2k may also be formed so that the leveled-up portion bulges convexly inthe radially outward direction from the reference line SL as shown inFIGS. 5(b) and 5(d). This enables the bending resisting strength of theinsulator 2 to be further improved. Especially, as shown in FIG. 5(d),forming a projecting rounded portion on an intermediate section of thecontour line of the built-up portion 2 k, i.e., forming an apex sectionof a projecting contour line in a moderately rounded shape, enables thestress scattering effect to be further improved. In addition, formingthe built-up portion 2 k in such a shape is also effective in preventingthe same portion (especially, in the form of a molded body not yetclamped) from being chipped.

EXAMPLES

[0062] In order to ascertain the effect of the present invention, thefollowing tests were conducted.

[0063] Embodiment 1:

[0064] Spark plug specimens in which the sizes shown in FIG. 3 and FIG.4 were set as follows were produced (the parameters already describedwere shown by their symbols only).

[0065] Outer diameter F of the intermediate trunk portion 2 g: 7.3 mm

[0066] Outer diameter T of the expanded diameter portion 1 e: 11 mm

[0067] Angle Q: 30°

[0068] Value of B≡(U−F)/2 wherein a represents the position of aboundary between a rounded portion formed between an inclined portion 2j and a built-up portion 2 k and the inclined portion 2 j; and U anouter diameter of an insulator 2 in the position a: 0.9 mm

[0069] Cross-sectional area S1: 52 mm²

[0070] Cross-sectional area S2: 30 mm²

[0071] Width M: 0.64 mm

[0072] Distance C in the axial direction between intersection n andintersection J: 1 mm

[0073] Distance W: 3 mm (C/W=3)

[0074] Distance E shown in FIG. 4: 33 mm

[0075] Distance D shown in the same drawing: 21 mm

[0076] Overlapping length P in the axial direction of the built-upportion 2 k on the intermediate trunk portion 2 g: 0.5 to 2.9 mm

[0077] The following tests were conducted using spark plugs in which thelength of the built-up portion 2 k was regulated variously as mentionedabove.

[0078] <1> Impact resistance test: As shown in FIG. 7, an attachingscrew portion 7 of each spark plug 100 is turned into a threaded hole303 a of a specimen fixing block 303, and fixed so that a main bodyportion 2 b of an insulator 2 projects upward from the hole 303 a. Anarm 301 is fixed to an axial fulcrum 302 positioned on the portion of acentral axis O of the insulator 2 which is still higher than the mainbody portion 2 b, such that the arm can be swung. The length of the arm301 is 330 mm, and the position of the axial fulcrum is determined sothat a position of a front end 300 of the arm swung downward to a rearside section of the main body portion 2 b is removed from a rear endsurface of the insulator 2 by a vertical distance H of 10 mm. Anoperation of lifting a front end of the arm so that an angle at whichthe arm 301 is swung from the central axis O thereof attains apredetermined level, and swinging the arm downward by free fallingtoward the rear side section of the main body portion 2 b is repeated atangle intervals of 2° which are increased gradually, to determine avalue θ of an impact resisting angle at which the bending of theinsulator 2 occurs. A larger angle θ means higher bending resistance(Charpy Strength).

[0079] <2> Screw neck strength test: A spark plug is fixed to a threadedbushing, and torsional torque is applied to the spark plug in thetightening direction with the fixing seat portion gripped, to determinea value of critical torque at which a screw neck portion is broken.

[0080] The results of plotting these test results with respect to P/Care shown in FIG. 8. The expression (1) mentioned above can betransformed to:

W/C≧P/C≧0.5  (1)

[0081] It is understood from the results of a comparison between theimpact resistance test results and the test results of FIG. 8 that, whenP/C is smaller than 0.5, the bending resistance becomes excellent, andthat, when P/C is not larger than 3 (i.e., not larger than W/C), thescrew neck strength becomes excellent.

[0082] Embodiment 2:

[0083] Spark plug specimens in which the sizes shown in FIG. 3 and FIG.4 were set as follows were produced (the parameters already describedare shown by their symbols only).

[0084] Outer diameter F of intermediate trunk portion 2 g: 7.3 mm

[0085] Outer diameter T of expanded diameter portion 1 e: 11 mm

[0086] Angle Q: 30°

[0087] Value B≡(U−F)/2 wherein a represents position of the boundarybetween the rounded portion formed between an inclined portion 2 j and abuilt-up portion 2 k and the inclined portion 2 j; and U the outerdiameter in the position a of an insulator 2: 0.9 mm

[0088] Cross-sectional area S1: 51.7 mm²

[0089] Cross-sectional area S2: 29.9 mm²

[0090] ((S1/E)/(S2/D) is varied in the range of 0.5 to 2.0)

[0091] Width M: 0.64 mm

[0092] Distance C in the axial direction between an intersection n andan intersection J: 1.1 mm

[0093] Distance W: 3 mm(C/W=3)

[0094] Distance E shown in FIG. 4: 33 mm

[0095] Distance D shown in the same drawing: 10 to 40 mm

[0096] Length P of overlap in the axial direction of a built-up portion2 k on intermediate trunk portion 2 g: 0 mm (Shape 1: ComparativeExample), 1.5 mm (Shape 2: Embodiments)

[0097] The same impact resistance tests as in Embodiment 1 wereconducted using spark plugs in which (S1/E)/(S2/D) were regulatedvariously as mentioned above. The results are shown in FIG. 9. Accordingto these results, the value θ of impact resistance angle decreasessuddenly when (S1/E)/(S2/D) becomes two or less in Comparative Example(Shape 1) in which a leveled-up portion is not formed, while the value θof impact resistance angle is large even when (S1/E)/(S2/D) is less thantwo in the Embodiment in which a leveled-up portion is formed. It isunderstood that a remarkable difference in bending resistance occursbetween the specimens of the Embodiment and those of the ComparativeExample.

[0098] It should further be apparent to those skilled in the art thatvarious changes in form and detail of the invention as shown anddescribed above may be made. It is intended that such changes beincluded within the spirit and scope of the claims appended hereto.

[0099] This application is based on Japanese Patent Application No.2001-366640, filed Nov. 30, 2001, the disclosure of which isincorporated herein by reference in its entirety.

What is claimed is:
 1. A spark plug provided with a shaft type centralelectrode (3), a shaft shaped insulator (2) covering the outer surfaceof the central electrode (3), a main metal member (1) which is formedcylindrically with both ends opened and which is disposed on an outerside of the insulator (2), and an earth electrode (4) joined at one endwith the main metal member (1) and the other end of the earth electrodebeing opposed to the central electrode (3) so as to form a sparkdischarge gap g, said spark plug having a center axis O in an axialdirection of the insulator (2), characterized in that: the main metalmember (1) is provided on its outer circumferential surface with anattaching screw portion (7) and a tool engaging portion (1 e) to enableturning the attaching screw portion (7) into a threaded hole in aninternal combustion engine, the length from side to side Σ of the toolengaging portion 1 e being not larger than 14 mm, the side in the axialdirection of the insulator 2 on which the spark discharge gap g isformed is defined as the front side and the opposite side thereof isdefined as the rear side, the insulator (2) is provided with a expandeddiameter portion (2 e) positioned in the main metal member (1) andprojecting radially outward from an outer circumferential surface of theinsulator (2), and an intermediate trunk portion (2 g) having acylindrical outer circumferential surface, formed adjacently in theaxial direction to a front side of the expanded diameter portion (2 e)and engaged at its front end with a main metal member-side engagingsection (1 c) formed on an inner circumferential surface of the mainmetal member (4), an outer circumferential surface of an inclinedportion (2 j) forms a linear incline in a position connecting theexpanded diameter portion (2 e) and intermediate trunk portion (2 g),such that the diameter of the inclined portion (2 j) decreases from theexpanded diameter portion (2 e) toward the intermediate trunk portion (2g) in orthogonal projections with respect to a plane of projectionparallel to said axis, a built-up portion (2 k) being formed where anextension of the outer circumferential surface of the inclined portion(2 j) and that of the intermediate trunk portion (2 g) cross each other,such that a valley-like space defined by these extensions is filled withthe built-up portion (2 k).
 2. The spark plug as claimed in claim 1,wherein an outer circumferential surface of the built-up portion (2 k)has a concave rounded section formed where the built-up portion isconnected to the outer circumferential surface of the inclined portion(2 j) and to the outer circumferential surface of the intermediate trunkportion (2 g).
 3. The spark plug as claimed in claim 1, wherein theouter circumferential surface of the built-up portion (2 k) is in aposition on or bulged radially outward from a reference line formed insaid orthogonal projection by a straight line connecting together apoint A in which the outer circumferential surface of the built-upportion (2 k) and that of the inclined portion (2 j) are connectedtogether and a point c in which the outer circumferential surface of thebuilt-up portion (2 k) and that of the intermediate trunk portion (2 g)are connected together.
 4. The spark plug as claimed in claim 1, whereinsaid orthogonal projection, a projecting rounded portion is formed on anintermediate section of a contour line of the built-up portion (2 k). 5.The spark plug as claimed in claim 1, wherein the width of the outercircumferential surface of the inclined portion (2 j) is not smallerthan 0.3 mm and not larger than 3 mm, an angle between a plane Q of saidorthogonal projection crossing the axis at a right angle and the outercircumferential surface of the inclined portion 2 j being not largerthan 60°.
 6. The spark plug as claimed in claim 1, wherein a flangeshaped fixing seat portion (1 g) is formed on the part of the outercircumferential surface of the main metal member (1) between the toolengaging portion (1 e) and attaching screw portion (7), and the relationW≧P≧0.5C is satisfied, wherein, in the orthogonal projection, a firstintersection J is the intersection of where the extension of thecylindrical outer circumferential surface of the expanded diameterportion (2 a) and that of the inclined portion (2 j) cross each other; asecond intersection n is the intersection of the extension of the outercircumferential surface of the inclined portion (2 j) and that of theouter circumferential surface of the intermediate trunk portion (2 g); cis the point at which the outer circumferential surface of the built-upportion (2 k) and that of the intermediate trunk portion (2 g) areconnected together; W is the distance in the direction of the axis fromthe front end (1 i) of the fixing seat portion (2 g) to the secondintersection n; P is the distance from the first connecting point c tothe second intersection n; and C is the distance from the secondintersection to the first intersection J.
 7. The spark plug as claimedin claim 1, wherein an outer diameter of the intermediate trunk portion(2 g) is not smaller than 5 mm and not larger than 8 mm.
 8. The sparkplug as claimed in claim 1, wherein a main body portion (2 b) having acylindrical outer circumferential surface the diameter of which issmaller than that of the expanded diameter portion (2 e), is formed onthe insulator so that the main body portion (2 b) adjoins the axial rearsection of the expanded diameter portion (2 e), and the relation0.5≦(S1/E)/(S2/D)≦2 is satisfied, wherein, in orthogonal projection, Krepresents the position of the midpoint of an axial line connecting bothends of the cylindrical outer circumferential surface of the expandeddiameter portion (2 e); T represents the position of the rear end in theaxial direction of the outer circumferential surface of the insulator(2); S represents the position of the front end in the axial directionof the cylindrical outer circumferential surface of the intermediatetrunk portion (2 g); E represents a distance measured in the axialdirection from the position T of the rear end to the position K of themiddle point; S1 represents the axial cross sectional area of the mainbody portion (2 b); and S2 represents the axial cross sectional area ofthe intermediate trunk portion (2 g).